1. Field of the Invention
The invention relates generally to the field of telephony switching systems, and more particularly to such systems for use in cellular mobile telephone systems. The invention provides a new and improved system that includes mechanisms both for connecting to and controlling an unregulated cellular system and for connecting to a regulated public telephony network so that the same system can perform both functions and be sufficiently divided to insure that the unregulated cellular portion is not subsidized by the regulated landline portion.
2. Description of the Prior Art
Cellular radio-telephone systems have recently been introduced in a number of areas to provide telephone coverage for people who need to have access to telephones for communications, but who must be outside of their offices for long periods of time and are otherwise unable to get to elephones that are hard-wired to a central office over conventional telephone lines. Users of cellular systems may include, for example, people of any of a number of occupations, such as salesmen, repairmen, or delivery men, who mus do considerable amounts of automobile travel and who may not be near a conventional land-line telephone when their supervisors or dispatchers may need to communicate with them.
In the past, radio-telephone systems have provided limited and expensive service to a number of areas. In prior systems, a radio signal from a single high-power transmitter covered an entire area. The number of subscribers who could use the system at any one time was limited by the number of channels provided for radio-telephone service, which, in turn, was limited by the amount of the radio frequency spectrum that was allocated to radio-telephone usage in the area and the bandwidth of each channel. In most prior radio-telephone systems, the number of channels, and thus the number of subscribers in a region who could use the system at any one time was also small.
In cellular radio-telephone services, however, an aras is divided into a plurality of small regions, or "cells", covered by a low-power transmitter. Currently, cellular radio-telephone service is provided in frequency bands between 825 and 845 MHz and between 870 and 890 MHz. The higher frequency band is used for down-link transmissions, that is, transmissions from the "cell site" for reception by a mobile subscriber. The "cell site" is the location of the radio frequency transmitter, or, more specifically, the location of the antenna, from which transmissions are effected for the cell. The lower frequency band is used for up-link transmissions, that is, transmissions from the subscriber for reception by the receiving equipment which is also located at the cell site.
Each of the frequency bands allocated to cellular radio-telephone service in an area is divided into two parts, with one-half being reserved for the local land-line telephone company and the other half being franchised to a competing radio-telephone service provider. Each channel has a bandwidth of thirty kilohertz, allowing for 666 channels in the twenty-megahertz bands, with 333 being provided to the telephone company and the same number being provided to the franchisee. Most of the channels are used for voice transmission, but some, including at least one channel in each cell, are used as a paging/access channel, which is used for transmission of control information between the cell site and mobile subscribers.
To avoid interference between transmissions in adjacent cells, the entire twenty megahertz bandwidth is not available in all of the cells. Instead, cells are assigned certain of the channels, such that adjacent cells are not assigned the same channels. Typically, the cells may be arranged so that each cell is surrounded by six others, and so each cell may have, for example, forty-eight channels provided by each of the telephone company and franchisee (that is, 333 channels divided by seven). The actual topography of the cells and number of channels in the various cells may vary depending on a number of factors. As subscribers travel between cells, the channels in which they transmit and receive the telephonic voice signals are changed in a manner and by circuitry known in the art. Thus, ninety-six simultaneous calls can take place in each cell, one over each of the channels. Using prior radio-telephone arrangements, with the same bandwidth signals and bands, only ninety-six calls could take place in an entire area.
Currently, the billing rates for land-line telephone services throughout the United States are regulated by various state and local departments of public utilities. These rates depend, in part, on the amount of capital equipment that has been dedicated by the telephone company for use in the public switching network for the landline subscribers. However, the rates for cellular radio-telephone service are not regulated, and so the departments of public utilities require that the telephone companies maintain a rigid demarcation between the investments for the regulated activities and their investments in the unregulated areas such as cellular radio-telephone services. This is required to make sure that they do not include in their regulated rate requests investments that are actually used for their unregulated activities.